The Luas Cross city project is an extension of the Green Line which will link it with the Red Line, and continue northwards to Broombridge in North Dublin (interchange with Irish Rail station). The extension will begin at the existing St.Stephen's Green Green Line stop. Construction started in June 2013 with services expected to begin in late 2017.

Line BX (includes Line D to Broombridge) – City Centre link for Red and Green Lines. The RPA started public consultation on the route in December 2005. In March 2007 the preferred route was announced. The planned route runs from St. Stephens Green to College Green where the line changes from a double track to single track. From here it runs north through Westmoreland St., over O'Connell Bridge and along the west side of O'Connell St. to Cathal Brugha St. It then turns east into Cathal Brugha St. and turns south to run along Marlborough St., across the River Liffey on a new bridge, continues along Hawkins St. and College St. and joins up with the double track section of the line at College Green. 2012 was the original completion date given in the Transport 21 plans, but construction has only started in 2013. The completion date is 2015–2016 with passenger services to commence in 2017.

The RPA applied for a Railway Order application to An Bord Pleanála for a combined Line D / Line BX Luas Line that is planned to run from St. Stephen's Green to Broombridge via the city centre and Broadstone / Grangegorman.

Line D – City Centre to Liffey Junction. This is expected to serve Grangegorman, the site of the proposed new DIT campus. This line will linked with the Maynooth line.

On 10 November 2011, the government announced in its 2012–16 Infrastructure and Capital Investment plan that the project to link the Red and Green lines, known as BXD, was to proceed. No other new lines or extensions are being funded.

Construction work for the bridge across the River Liffey (connecting Marlborough St and Hawkins St) began in April 2012, which the southbound Luas BXD track will be laid on. A Railway Order was granted by ABP for the Luas BXD line on 3 August 2012. The construction project has since been branded Luas Cross City.